The invention relates to an operating circuit for discharge lamps.
It relates here to operating circuits which supply the discharge lamp with radio-frequency supply power which is obtained from a supply power via an oscillator circuit. In particular, although not exclusively, the invention relates to the case in which the supply power for the oscillator circuit is obtained from an a.c. voltage supply power which is rectified. Operating circuits of this kind are in general use, in particular for low-pressure discharge lamps, and therefore need not be explained in detail.
The oscillator circuit in this case supplies what is known as a load circuit, into which the discharge lamp is connected, and through which a radio-frequency lamp current, generated by the oscillator circuit, flows. The load circuit defines in this case a resonant frequency which is influenced by various electrical parameters of the load circuit and is also dependent on, among other things, the operating state of the discharge lamp. The aim is to operate the load circuit relatively close to the resonant frequency during continuous operation of the discharge lamp. This has the advantage of small phase shifts between the current and voltage, and thus small reactive currents. This is of benefit when dimensioning components, in particular of a lamp inductor. Otherwise, the oscillator circuit which generates the radio-frequency supply power generally contains switching elements. When phase shifts are small due to operation close to resonance, the switching losses in the switching elements are relatively low. This has advantages with regard to the efficiency of the operating circuit as well as to the thermal load and the dimensioning of the switching elements.
The aim is normally to operate in what is known as the inductive region, i.e. at an operating frequency of the oscillator circuit which is greater than the resonant frequency of the load circuit. This does, however, require that the operating frequency of the oscillator circuit be prevented from falling below the resonant frequency, since, in capacitive operation, i.e. when the operating frequency is less than the resonant frequency, disturbing current spikes can be produced in the switching elements, or other problems may result. It is particularly possible in capacitive operation, due to the switching times and the lamp inductor current being incorrectly synchronized, for a pronounced positive current spike to be produced at the beginning of a lamp current half-cycle that is carried by a switching element. It is therefore aimed, on the whole, to operate as close as possible to the resonant frequency, but the frequency should not, if possible, fall below the resonant frequency, or should only fall below it to a limited extent.
However, fluctuations in the lamp impedance (based on continuous operation) occur as a result of temperature changes and aging processes such as electrode wear, mercury diffusion in fluorescent substances and other aging phenomena as well as scatter between the individual examples of different individual discharge lamps.
These lamp impedance fluctuations and the usual component tolerances mean that the operating circuits cannot easily be set relatively accurately to operation close to resonance. On the contrary, for reasons of safety, a relatively large margin is maintained from the nominal resonant frequency in order to take into account the fluctuations and tolerances mentioned. This results in increased component costs and increased space requirement due to the correspondingly larger dimensioning as well as in losses in efficiency.
Attempts have therefore already been made to equip operating circuits of the described construction with detection circuits for identifying the proximity to capacitive operation of the load circuit. For example, FIG. 5 of U.S. Pat. No. 6,331,755 shows a resistor RCS for measuring a lamp inductor current and a comparator COMP for comparing this inductor current with a threshold value. The comparison is carried out on a switching-off flank of a switching transistor in a half-bridge oscillator circuit. The closer the operating frequency comes to the resonant frequency, and therefore to capacitive operation, not only the smaller is a switching-on peak of the measurement voltage (at which the mathematical sign is reversed) across the resistor RCS, but also the more the measurement voltage falls at the end of the time for which said switching transistor is switched on. It is therefore possible to use the threshold value to set a limit state in which the circuit is completely switched off (shown on the right-hand side of FIG. 6 of that document) if operation becomes too close to resonance.
Against the background of the cited prior art, the technical problem on which the invention is based is to further improve an operating circuit for a discharge lamp having an oscillator circuit and a detection circuit for identifying the proximity to capacitive operation of the load circuit.
The invention relates to an operating circuit of the type described, in which the detection circuit detects the magnitude of fluctuations, corresponding to the changes in supply power, in the lamp current or in a manipulated variable of a lamp control circuit.
Preferred embodiments are given in the dependent claims.
The invention is characterized by the detection circuit identifying the proximity to capacitive operation in a particularly advantageous form. For this purpose, in a variant of the invention the detection circuit detects, the magnitude of fluctuations, corresponding to the frequency of the supply power, in the lamp current. If the oscillator circuit is supplied with a rectified a.c. voltage supply power, the supply power of the oscillator circuit fluctuates with the fluctuations, resulting from the a.c. voltage frequency, in the rectified supply voltage (what is known as the intermediate circuit voltage). The intermediate circuit voltage is therefore modulated at twice the frequency of the original a.c. voltage. It is the rectification process which causes the frequency to be doubled. It is theoretically also conceivable for no frequency doubling to occur here; in any case, the modulation of the intermediate circuit voltage is related to the frequency of the original a.c. voltage.
This intermediate circuit voltage modulation can generally still be measured in the lamp current itself, specifically even if the lamp current is determined by means of a current or power control circuit, which constitutes a preferred embodiment of the invention. Control circuits, depending on the technical complexity, are capable of attenuating this modulation only to a limited extent. If no control circuit is provided, it is even easier for the modulation of the intermediate circuit voltage to be identified in the lamp current.
Moreover, this also applies to the case, which likewise represents a preferred embodiment of the invention, in which the rectified a.c. voltage supply power is converted to a substantially constant d.c. voltage by means of a PFC (Power Factor Correction) circuit. The PFC circuit is used to limit the harmonic content of the power consumption from the a.c. voltage network and generally charges a storage capacitor to the intermediate circuit d.c. voltage. The intermediate circuit voltage is then also modulated, to a certain extent, in accordance with the a.c. voltage frequency.
The magnitude of the lamp current fluctuations depends on the proximity to the resonant frequency and thus on the proximity to capacitive operation. This follows from the increase in the lamp current with increasing proximity to resonance, on the one hand, and from the modulation of the proximity to resonance by the intermediate circuit voltage modulation, on the other hand.
The magnitude of the fluctuations in the lamp current is thus a particularly simple way of detecting the proximity to capacitive operation. Of particular concern here is a signal which varies, for example, at twice the mains frequency of the a.c. voltage network and which to this extent does not represent any substantial difficulties in terms of measurement. On the other hand, the conventional solutions for detecting the proximity to capacitive operation are linked to the operating frequency of the oscillator circuit itself and must be referred to these phases, which requires a considerably greater degree of circuitry complexity. The lamp current must in many cases be measured for other reasons anyway, for example in order not to exceed certain maximum values for safety considerations or in order to carry out the current regulation mentioned above. The invention is thus associated with even less additional outlay.
In the general description of the invention in claim 1 and claim 2, mention is made of a variable supply power. As mentioned above, this may, on the one hand, be a rectified a.c. voltage supply power. The invention does, however, also include the case in which the operating circuit is operated using a d.c. voltage source. In this case, there is no need for a rectifier, or any rectifier which is provided in any case has no effect. In this case too, however, it may also be desirable to use the invention. For this purpose, the d.c. voltage or intermediate circuit voltage may be modulated in a deliberate manner. In addition to the possibility of detection, according to the invention, of the proximity to capacitive operation of the load circuit, this also has the advantage that, as a result of the modulation, the frequency spectrum of radio-frequency interference, which is transmitted through the operating circuit to the d.c. voltage source, is broadened. The interference is thus less problematic since it occurs over a wider, and therefore flatter, interference spectrum. The variable supply powers, for the purposes of the claims, may therefore also be d.c. voltage supply powers which have been modulated in a deliberate manner. The invention particularly also relates to combination operating circuits which are provided for operation from both d.c. voltage and a.c. voltage sources.
As an alternative to detecting the magnitude of the fluctuations in the lamp current itself, the invention also aims at the case where the lamp current is determined by a control circuit for controlling the load circuit, i.e. in particular the lamp current or the lamp power, in which case a manipulated variable is detected for the control circuit, i.e. the changes in the control circuit when the control circuit is attempting to keep the controlled variable constant. The manipulated variable could then be regarded as an image of the lamp current fluctuations, even if the lamp current fluctuations are not occurring, or occurring only to a limited extent.
The control circuit preferably has an I control element, i.e. an integrating element, in order to compensate for the comparatively slow parameter changes in the discharge lamp in terms of the described changes in impedance due to aging or other long-term fluctuations. In many cases, such an I control element will be sufficient. If required, it may be supplemented by a P control element (proportional element) or by some other additional device in order to take better account of the intermediate circuit voltage modulation.
In particular, the control circuit and other means of controlling the oscillator circuit may be provided by means of an integrated digital circuit which has to have only a few additional functions. Furthermore, the digital circuit may be a programmable circuit or what is known as a microcontroller, in which case the additional complexity required for the invention may be limited just to additional software.
Such a digital control circuit or such a microcontroller may, in particular, in addition to controlling the oscillator circuit, also adopt the function of controlling the PFC circuit mentioned.
It is preferably also provided for the operating circuit not to be switched off when specific proximity to capacitive operation is identified, as is the case in the prior art, but for its operation to be continued, at least normally. Identification of the proximity to capacitive operation should therefore result in the method of operation being influenced such that this proximity is at least increased no further or is even reduced, making it possible to continue operation. For example, the operating frequency of the oscillator circuit could be directly influenced. The preferred solution for the case of a control circuit is, however, to reduce the desired current value or the desired power value of the current control circuit, which may cause the frequency to be indirectly influenced. To clarify, the operating circuit according to the invention is thus designed not to come too close to capacitive operation during continuous operation and to prevent it getting any closer to capacitive operation if it is already too close, but with lamp operation continuing. For this purpose, it is acceptable, in particular, to change parameters which may have been predetermined in a fixed manner, such as the operating frequency or the lamp current, if necessary. Specifically, from the point of view of the invention, it would be more acceptable for the discharge lamp to dim slightly in situations such as this than to be switched off entirely.
It may be provided, in particular, for the detection circuit to compare the magnitude of the fluctuations with a predetermined threshold value and, as long as the threshold value is not exceeded, to influence operation no further. If the threshold value is exceeded, the detection circuit may either continuously vary the operating frequency, the desired control value or another variable in accordance with a control context, or else vary it by a predetermined fixed amount, as illustrated in the exemplary embodiment. In any case, the comparison with the threshold value preferably results in a detection circuit function which does not normally influence operation.